CO 
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CJ 


No.  1797 


HANDBOOK  OF 

RANGE-FINDERS 

70  CM.  AND  80  CM.  BASE 

FOR  USE  OF  INFANTRY  AND  CAVALRY 

-w 

WITH  DESCRIPTION  AND  INSTRUCTIONS 
FOR  THEIR  CARE  AND  USE 


(TWELVE  PLATES) 


DECEMBER  9,  1915 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1917 


No.  1797 


HANDBOOK  OF 


RANGE-FINDERS 

70  CM.  AND  80  CM.  BASE 

FOR  USE  OF  INFANTRY  AND  CAVALRY 


WITH  DESCRIPTION  AND  INSTRUCTIONS 
FOR  THEIR  CARE  AND  USE 


(TWELVE  PLATES) 


DECEMBER  9,  1915 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1917 


WAR  DEPARTMENT, 
OFFICE  OF  THE  CHIEF  OF  ORDNANCE, 

Washington,  December  9,  1915. 

This  manual  is  published  for  the  information  and  government  of  the  Regular  Army 
and  Organized  Militia  of  the  United  States. 
By  order  of  the  Secretary  of  War: 

WILLIAM  CROZIER, 
Brigadier  General,  Chief  of  Ordnance* 
(3) 


364469 


CONTENTS 


Page. 

Self-contained  base  range-finders 7 

Goerz  range-finder  (80  cm.  base) 13 

Bausch  &  Lomb  range-finder  (70  cm.  base) 17 

Bausch  &  Lomb  range-finder  (80  cm.  base) 21 

Care  and  preservation 25 

(5) 


SELF-CONTAINED  BASE  RANGE-FINDERS. 


GENERAL  PRINCIPLE. 

x 

If  the  length  of  one  side  and  the  value  of  any  two  angles  of  a 
triangle  are  known,  the  length  of  the  other  two  sides  of  the  triangle 
can  be  calculated. 


ANGLE  TO  BE 
MEASURED. 


Fig.l.  .Fig.  2 

Thus,  in  figure  1,  if  the  length  of  the  side  AB  and  the  angles  BAD 
and  DBA  are  known,  the  remaining  sides  AD  and  BD  can  be  calcu- 
lated. 

If  the  length  of  the  side  AB  is  fixed  and  the  angle  BAD  remains  a 
right  angle,  then  it  is  only  necessary  to  measure  the  angle  DBA  in 
order  to  be  able  to  calculate  the  lengths  of  the  sides  AD  and  BD. 
It  will  be  seen  from  figure  1  that  the  longer  the  side  AD  becomes 

(7) 


8 

(as  ADr),  the  larger  the  angle  DBA  becomes  (as  D'BA),  and  that 
for  any  given  length  of  AD  there  will  always  be  a  corrsponding 
value  of  the  angle  DBA. 

Suppose,  therefore,  that  a  range-finder  be  constructed  by  placing 
a  telescope  at  the  point  A  and  so  arrange  it  that  its  axis  or  line  of 
sight  makes  a  right  angle  with  the  line  AB.  Place  another  telescope 
at  the  point  B  and  so  arrange  it  that  the  angle  DBA  which  its  lino 
of  sight  makes  with  the  line  AB,  can  be  measured.  Then  if  the  line 
AB  is  known  and  remains  fixed,  the  length  of  the  sides  AD  or  the 
range  of  the  object  D  from  A  can  be  computed.  (See  fig.  2.) 

A  table  can  be  computed  giving  the  length  of  the  side  AD  of  the 
triangle  ABD  for  all  values  of  the  angle  DBA  from  0°  to  90°  for  a 
fixed  length  of  base  AB.  Then  instead  of  graduating  the  angle- 
measuring  scale  on  the  telescope  at  B  in  degrees,  graduate  it  with 
its  corresponding  values  of  the  lengths  of  the  side  AD.  We  now 
have  a  range-finder  which  by  measuring  the  angle  DBA  with  the 
telescope  at  B  will  read  directly  the  range  of  the  object  D  from  A. 

To  use  such  a  range-finder,  the  observer  at  the  telescope  A  would 
lay  his  telescope  on  object  D,  whose  range  from  A  is  desired.  As  the 
telescope  A  is  rigidly  connected  at  right  angles  to  the  base  line  AB, 
the  whole  apparatus  together  with  the  telescope  at  B  would  be  moved 
at  the  same  time.  The  observer  at  B  would  then  turn  his  telescope 
until  the  image  of  D  coincided  with  the  cross  wire.  The  range  AD 
could  then  be  read  directly  from  the  angle  scale  on  the  telescope  at  B. 

The  single  observer  infantry  and  cavalry  range-finder  goes  a  step 
further  and  by  means  of  reflexing  prisms  or  mirrors  permits  a  single 
observer  to  see  the  images  from  both  telescopes  A  and  B  in  a  single 
eyepiece  located  midway  between  A  and  B.  An  observer  looking 
into  this  eyepiece  sees  the  field  divided  into  two  parts  by  a  hori- 
zontal line.  In  the  lower  part  of  the  field  the  observer  sees  the 
image  from  the  telescope  at  A  and  in  the  upper  part  of  the  field  the 
image  from  the  telescope  at  B.  There  are  other  types  of  range- 
finders  in  which  the  field  is  divided  into  three  parts. 

In  the  single  observer  infantry  range-finder  when  the  telescope 
at  B  is  turned,  an  object  in  the  upper  field  observed  through  the. eye- 
piece appears  to  move  laterally  across  the  field. 

Plate  I  gives  a  diagrammatic  sketch  of  a  self-contained  base, 
single  observer  range-finder. 

A  study  of  Plate  I  will  show  that  the  instrument  is  equivalent  to 
two  telescopes.  On  the  left  side  is  the  fixed  telescope  corresponding 
to  the  telescope  at  A  in  the  improvised  range-finder  just  described. 
In  the  left  telescope  the  light  traverses  the  objective  lens,  the  coin- 
cident prism  D  and  the  eyepiece  consisting  of  the  field  lens  G  and 
the  eyelens  F.  The  line  of  sight  of  the  left-hand  telescope  is  turned 


through  90°  by  means  of  the  reflecting  prism  A.  At  the  right-hand 
side  is  the  movable  telescope  corresponding  to  the  telescope  at  B  in 
figure  2  of  our  improvised  range-finder.  In  the  right-hand  telescope 
the  light  traverses  the  objective  lens,  the  angle  measuring  prism  P, 
the  coincident  prism  D',  and  the  same  eyepiece  (G  and  F).  As  before, 
the  line  of  sight  of  the  right-hand  telescope  is  turned  through^QO0  by 
means  of  the  reflecting  prism  B. 

Instead  of  moving  the  telescope  at  the  right-hand  side  mechani- 
cally, in  order  to  measure  the  angle  DBA  (fig.  1),  the  line  of  sight  of 
the  telescope  is  moved  optically  by  means  of  the  reflecting  prism  P. 
This  prism  is  sometimes  called  the  angle-measuring  prism  or  wedge. 
The  reflecting  prism  P  carries  a  range  scale  which  is  graduated 
directly  in  yards  of  range.  Thus,  moving  the  reflecting  prism  P 
laterally  along  AB  corresponds  to  a  movement  of  the  line  of  sight  of 
the  right-hand  telescope,  and  the  angle  which  the  line  of  sight  of 
this  telescope  makes  with  the  line  AB  (fig.  1)  is  read  in  terms  of  the 
range  AD  (fig.  1)  on  the  range  drum  attached  to  the  reflecting  prism. 

Referring  to  Plate  I,  if  CA  and  C'B  are  parallel  rays  of  light  from 
an  object  at  an  infinite  distance,  they  will  meet  at  the  center  D.  If 
the  object  is  at  the  finite  C  and  C",  then  CA  will  be  reflected  to  D 
as  before,  but  C"B  will  be  reflected  to  D'.  The  angle  D'BD  is  equal 
to  the  angle  C'BC"  and  varies  with  the  distance  of  the  observed 
object.  The  ray  BD'  could  be  made  to  take  the  direction  of  BD  by 
revolving  the  reflector  B,  but  the  amount  of  this  revolution  is  so 
small  for  moderate  changes  in  range  and  so  difficult  for  accurate 
measurement  that  the  same  result  is  obtained  by  the  use  of  the 
prism  P,  the  reflecting  angle  of  which  is  very  small. 

A  position  of  the  reflecting  prism  P  can  be  found  varying  with  the 
distance  of  the  observed  object  where  the  reflected  ray  will  pass 
through  the  point  D,  thus  meeting  the  ray  CAD.  The  two  coincid- 
ing images  of  the  same  prisms  are  so  placed  at  the  point  D  that  they 
reflect  the  light  from  both  ends  of  the  range-finder  into  the  eyelens 
F,  and  the  images  are  actually  found  at  a  point  in  front  of  the  eye- 
lens,  the  position  of  the  images  depending  upon  the  type  of  the 
instrument.  Since  there  is  a  certain  position  of  the  prism  P  which 
will  bring  the  two  halves  of  the  object  into  coincidence,  then  a  suit- 
ably graduated  scale  attached  to  the  prism  and  moving  with  it  will 
record  the  distance  of  the  object  corresponding  to  the  position  of 
the  prism. 

In  the  actual  range-finder  various  prism  and  lens  combinations  are 
employed  in  the  place  of  the  simple  parts  shown  in  Plate  I,  in  Older 
to  obtain  the  desired  erect  or  invert  appearance  of  the  partial  images 
in  the  field  of  view  (see  Plate  IV).  All  the  optical  elements  are 
inclosed  in  a  tube.  As  the  tube  is  liable  to  be  distorted  as  a  result 
105239—17 2 


10 

of  the  effect  of  temperature  changes,  or  of  handling  during  the  work- 
ing of  the  instrument,  it  is  necessary  to  adopt  a  type  of  end  reflector 
that  will  not  alter  the  direction  of  the  beams  of  light  entering  the 
range-finder  even  if  its  position  were  varied  by  small  amounts. 
Nearly  all,  except  the  earlier  range-finders,  are  made  with  a  prism, 
known  as  the  pentagonal  or  Prandl  prism,  to  take  the  place  of  the 
reflectors  A,,  and  B  on  Plate  I. 

A  prism  of  this  type,  designed  to  bend  the  rays  of  light  through 
an  angle  of  90  degrees,  consists  of  a  block  of  glass  having  two  reflect- 
ing faces  placed  at  an  angle  of  45  degrees  to  one  another.  It  is  neces- 
sary that  these  two  reflecting  faces  should  be  silvered,  as  the  angle 
of  incidence  is  within  the  critical  angle  of  glass.  Any  rotation  of 
such  a  prism  about  an  axis  perpendicular  to  the  column  normal 
plane  of  the  two  reflecting  surfaces  does  not  affect  the  direction  of 
the  double  reflecting  beam  of  light,  as  the  effect  of  the  rotation  of  one 
reflecting  surface  is  exactly  balanced  by  the  equal  rotation  of  the 
two  reflecting  surfaces. 

Instead  of  using  an  angle-measuring  prism  like  P  (Plate  I) ,  which 
slides  horizontally  along  the  base  of  the  range-finder,  two  rotating 
prisms  are  often  used.  These  prisms  are  rotated  in  opposite  direc- 
tions by  gearing  connected  to  the  range  drum. 

The  actual  arrangement  of  the  optical  parts  used  in  the  Goerz 
80  cm.  base  range-finder  are  shown  on  Plate  IV. 

TYPES  OF  RANGE-FINDERS. 

There  are  several  different  types  of  self-contained  range-finders, 
three  of  which  are  described  in  this  pamphlet,  namely,  the  invert 
single-coincidence,  the  invert  double-coincidence,  and  the  erect 
double-coincidence  types. 

All  the  80  cm.  base  Goerz  range-finders  issued  to  the  service  are  of 
the  invert  single-coincidence  type.  The  Bausch  &  Lomb  instru- 
ments, 70  and  80  cm.  base  are  of  the  invert  double-coincidence  type 
except  1 — 70  cm.  base  instrument  No.  14  which  is  of  the  erect  double- 
coincidence  type. 

In  the  invert  single-coincidence  type  the  entire  upper  field  is 
inverted  but  not  reversed,  right  or  left,  giving  the  effect  as  though 
the  lower  field  was  reflected  in  the  sky. 

In  the  invert  double-coincidence  type  the  narrow  strip  in  the  center 
of  the  field  is  inverted,  the  upper  field  remaining  erect.  This  type 
gives  a  greater  effective  field  for  observation  and  permits  coincidence 
to  be  made  on  two  edges  of  the  invert  field  when  the  target  has  a 
vertical  straight  line,  as  in  a  chimney  or  edge  of  a  building.  Its 
optics,  however,  are  more  complex. 


11 

The  erect  double-coincidence  type  is  similar  to  the  invert  double- 
coincidence  type  except  that  the  image  in  the  central  narrow  strip 
is  erect. 

GENERAL  INSTRUCTIONS. 

The  best  denned  part  of  an  object  should  be  selected  to  range  on. 

Coincidence  should  always  be  made  in  the  center  of  the  field  of 
view  and  if  possible  with  the  dividing  line  at  right  angles  to  the 
object  observed. 

Upon  the  accuracy  of  the  coincidence  setting  depends  the  accuracy 
of  the  range  determination.  No  difficulty  should  be  experienced  on 
the  part  of  the  observer  in  obtaining  good  results  after  a  little  practice 
or  even  at  the  first  attempt,  but  in  range  finding  as  in  anything  else 
accuracy  increases  with  practice. 

It  has  been  found  that  different  observers  have  what  might  be 
called  personal  errors  which  diminish  as  their  experience  increases. 
The  range  observer  should  therefore  be  given  every  opportunity  for 
practice  in  measuring  angles  and  ranges,  and  in  making  the  halving 
adjustments.  When  both  edges  of  the  object  are  equally  well  defined 
the  observer  should  always  make  his  coincidence  on  the  same  edge, 
right  or  left.  Individual  observers  have  obtained  better  results  by 
so  doing  than  if  they  used  the  right  edge  sometimes  and  the  left  edge 
at  other  times.  It  is  advisable  when  time  permits  to  obtain  the 
range  by  taking  the  mean  of  several  consecutive  readings. 

With  targets  moving  laterally  across  the  field  it  is  best  to  adjust 
the  images  on  the  edge  from  which  the  targets  are  moving,  and  to 
make  the  measurements  for  range  while  the  target  runs  across  the 
field. 

Before  observing  range,  the  adjustments  for  height  and  for  distance 
should  be  tested,  and  corrected  if  necessary.  Adjustment  for  height 
should  always  be  made  before  adjustment  to  correct  for  errors  in 
distance.  See  the  descriptions  of  the  different  range-finders  for  the 
methods  of  making  the  adjustments  for  height  and  distance. 


GOERZ  RANGE-FINDER. 

(80  CM.  BASE.) 


EQUIPMENT  AND  GENERAL  DESCRIPTION. 

The  instrument  (Plates  V,  VI,  and  XII)  consists  of  the  range- 
finder,  the  tripod,  the  adjustment  bar,  the  accessories,  and  the  carry- 
ing case. 

The  principal  parts  of  the  range-finder  proper  are  the — 
A. — Eyepiece. 
B.— Finder. 
C. — Measuring  screw. 
D. — Range  scale. 

E.— Protective  cap  for  height  adjustment'screw. 
F. — Protective  cap  for  range  adjustment'screw. 
K. — Entrance  apertures. 
K.' 

L.—  Locking  hook. 

The  tripod  consists  principally  of  the — 
M.— Hook  bolt. 
N. — Legs. 

O. — Metal  tubing  extension. 
P. — Locking  screws  for  O. 
Q. — Locking  lever  for  N. 
R. — Casing. 

S. — Locking  screw  for  spur. 
T.— Spur. 

U. — Locking  screw  for  pivot. 
V. — Locking  screw  for  tilting  joint. 
W.— Elevation  screw. 
The  accessories  are  comprised  of— 
X. — Adjustment  bar. 
Y. — 2  color  screens. 
Z.— 2  sun  shades. 

1  piece  shamois  skin. 
1  brush. 
Optical  data: 

Invert  single-coincidence  type. 
Magnifying  power,  10. 
Actual  field  of  view  in  degrees,  4°. 
Entrance  pupil,  1  inch. 
Exit  pupil,  0.1  inch. 

Shortest  distance  measurable,  400  yards. 

(13) 


14 


Plate  IV  shows  the  optical  elements  mounted  in  the  same  relative 
position  as  in  the  instrument. 

The  accuracy  of  the  instruments,  even  under  the  most  favorable 
conditions,  will  be  unavoidably  influenced  by  a  known  error.  In  the 
following  table  the  lowest  values  of  these  errors  are  given,  but  when 
the  air  is  unsteady  or  the  target  is  unfavorable,  these  errors  may  be 
increased  five  times. 

Yards. 

900  yards 4.  9 

1,000  yards 6 

1 , 500  yards 12.5 

2,000  yards 23.8 

3,000  yards 54 


Yards. 

400  yards 1 

500  yards 1.  25 

600  yards 2 

700  yards 3 

800  yards 3.  8 


OPERATION.  , 

This  type  of  range-finder  may  be  used  in  the  prone,  kneeling,  or 
standing  position.  The  spur  (T),  Plate  VI,  is  used  as  a  support  for 
the  range-finder  when  in  a  prone  position .  Where  the  soil  is  soft  the 
spur  is  pushed  as  one  piece  vertically  into  the  ground.  For  use  on 
hard  soil  the  three  parts  composing  the  spur  are  spread  and  used  as  a 
short  tripod.  For  use  with  the  tripod  the  spur  is  folded  together, 
inserted  into  the  casing  R  of  the  tripod,  and  locked  securely  by  locking 
screw  S.  The  instrument  can  also  be  used  in  the  prone  and  kneeling 
positions  by  supporting  it  with  the  hands. 

For  use  in  standing  position  the  tripod  legs  must  first  be  lengthened 
by  loosening  locking  screws  P  and  allowing  metal  tubing  extension 
O  to  drop  out,  after  which  locking  screws  P  are  again  tightened.  If 
necessary,  the  spur  T  can  be  adjusted  in  the  casing  R  to  suit  the 
height  of  the  observer's  eyes. 

Take  the  range-finder  from  the  carrying  case,  with  eyepiece  turned 
toward  the  observer,  and  seat  the  instrument  firmly  on  the  tripod. 
Remove  the  protective  hoods  from  the  entrance  apertures  and  put 
on  sun  screens  Y  if  necessary,  turn  range-finder  toward  the  target 
and  fasten  locking  screw  U,  release  locking  screw  V,  turn  range- 
finder  so  that  its  longitudinal  axis  is  horizontal  and  relock  screw  V. 
Dismounting  is  accomplished  in  the  reverse  order. 

ADJUSTMENTS. 

A.  ADJUSTMENT  FOR  HEIGHT. 

When  in  sighting  at  an  object  the  partial  images  do  not  touch  the 
dividing  line  with  similar  points,  so  that  one  image  reaches  the 
dividing  line  before  the  other  (fig.  6,  Plate  II),  the  instrument  is  not 
in  adjustment  for  height. 

To  correct  this  error  the  range-finder  must  be  trained  at  an 
object  having  a  sharply  defined  horizontal  line  or  particularly  promi- 


15 

nent  point,  and  the  images  brought  laterally  exactly  opposite  each 
other  by  means  of  the  measuring  screw.  The  protective  cap  E, 
Plate  VI,  is  then  raised  and  by  means  of  the  height-adjustment 
screw  the  images  of  the  clearly  defined  line  or  prominent  point,  in 
both  the  upper  and  lower  fields,  are  brought  equidistant  from  the 
dividing  line.  Having  made  this  adjustment,  test  it  by  turning  the 
elevating  screw  W  until  similar  points  just  touch  the  dividing  line 
(see  fig.  5,  Plate  II).  The  adjustment  made,  the  protective  cap  is 
replaced  to  guard  against  unauthorized  or  accidental  shifting. 

B.    ADJUSTMENT  FOR  DiSTANCE. 

The  adjustment  for  the  correction  of  errors  in  distance  is  made 
with  the  adjustment  bar,  Plates  V  and  VI.  This  bar  is  set  up  not 
less  than  100  yards  from  the  range-finder  and  directly  facing  it.  The 
adjustment  bar  is  provided  with  av  finder,  by  means  of  which  the 
axis  of  the  bar  can  be  placed  at  right  angles  with  the  line  joining  the 
range-finder  and  the  center  of  the  adjustment  bar.  Both  ends  of 
the  adjustment  bar  are  marked  with  a  black  line.  The  distance 
between  these  lines  is  the  same  as  that  between  the  centers  of  the 
entrance  apertures  of  the  range-finder.  The  targets  are  made  to 
coincide  as  shown  in  figure  9,  Plate  III,  by  means  of  the  measuring 
screw  (C)  and  the  reading  of  the  range  scale  noted. 

This  should  be  repeated  several  times  to  insure  accuracy,  and  if 
the  average  reading  does  not  show  infinity  a  readjustment  must  be 
made.  This  adjustment  is  made  by  bringing  the  infinity  setting  of 
the  range  drum  opposite  the  index  and  making  coincidence  of  the 
targets  by  means  of  the  range-adjustment  screw  (F),  Plate  VI.  To 
insure  accuracy  coincidence  should  be  made  several  times,  noting 
the  readings  of  the  scale  on  the  range-adjustment  screw.  The  final 
adjustment  should  then  be  made  by  setting  the  range-adjustment 
screw  at  its  average  setting.  The  protective  cap  should  then  be 
closed  and  secured  to  prevent  accidental  or  unauthorized  disarrange- 
ment. 

MEASURING   RANGES. 

The  eyepiece  is  provided  with  an  adjustment  for  individual  cor- 
rection, equipped  with  a  diopter  scale.  If  this  correction  is  not 
known  it  should  be  determined  before  any  attempt  is  made  to 
measure  ranges.  The  determination  of  the  individual  correction  is 
made  by  turning  the  eyepiece  until  the  details  appear  sharply  defined 
in  the  field  of  the  range-finder.  The  resulting  number  on  the  diopter 
scale  should  then  be  memorized  for  future  use.  The  color  screen  is 
provided  to  protect  the  eyes  of  the  observer  from  a  glare  of  light. 

The  range-finder  having  been  set  approximately  on  the  target  (if 
necessary,  by  means  of  the  open  sight),  the  object  is  looked  for  in 


16 

the  field  of  view.  Movement  in  azimuth  is  accomplished  by  turning 
the  range-finder  on  the  tripod  pivot  after  loosening  locking  screw 
(U),  (see  Plate  VI).  For  use  with  stationary  targets  this  screw  is 
relocked  after  the  instrument  is  properly  set.  The  movement  in 
elevation  is  accomplished  by  means  of  the  elevation  screw  (W), 
Plate  V. 

The  field  of  view  of  a  range-finder  of  the  single  coincidence  invert 
type  is  divided  into  two  parts  by  a  horizontal  line.  In  the  lower 
part  the  image  of  the  object  appears  upright,  while  in  the  upper  part 
it  is  inverted  (fig.  3,  Plate  II) . 

By  turning  the  height  adjustment  screw  the  images  are  lined  up 
so  that  the  proper  points  will  touch  the  dividing  line  (fig.  4,  Plate  II) . 

By  turning  the  measuring  screw  (C),  Plate  VI,  the  image  is  shifted 
laterally  until  similar  points  of  the  object  are  exactly  above  one 
another  (fig.  5,  Plate  II).  The  distance  is  then  read  on  the  range 
scale. 

With  targets  which  have  no  prominent  vertical  lines  or  points,  but 
show  horizontal  lines,  such  as  tops  of  heights,  entrenchments,  etc., 
the  distance  is  measured  on  the  horizontal  line  by  setting  the  instru- 
ment vertical  (fig.  10,  Plate  III).  For  this  purpose  the  locking 
screw  is  loosened  and  the  range-finder  turned  down  on  the  left  until 
vertical.  Measurements  are  taken  in  the  same  manner  as  described 
above.  The  image  first  appears  as  illustrated  in  figure  7,  Plate  II. 
By  turning  the  measuring  screw  the  partial  images  are  shifted  so 
that  the  horizontal  line  is  continued  exactly  on  the  dividing  line 
(fig.  8,  Plate  II). 


BAUSCH  &  LOME  RANGE-FINDER. 

(70  CM.  BASE.) 


EQUIPMENT  AND  GENERAL  DESCRIPTION. 

The  instrument  (Plates  VII  and  VIII)  consists  of  the  range-finder 
proper,  the  tripod,  the  accessories,  and  the  carrying  cases  (Plate  IX). 
The  principal  parts   of  the  range-finder  proper  and  the   tripod 
mount  are  the— 

A.- — Entrance  apertures. 

B. — Range  roller. 

C. — Revolving  collar  (covering  rollers). 

D.— Height-adjustment  roller. 
D'. — Range-adjustment  roller  (not  seen). 

E. — Eyepiece. 

F. — Window  for  illuminating  range  scale. 

G. — Window  for  illuminating  adjusting  marks. 

H.— "Air"  and  " field "  adjustment. 

I. — Locking  screw  for  pivot. 

J. — Releasing  plunger. 

K. — Elevation  screw. 

L. — Casing. 

M.— Spur. 

N. — Shoulder  strap. 

P. — Eyepiece  cover. 

R. — Range-adjustment  levers. 

S. — Reference  scale. 

T.— Open  sight. 

V.- — Locking  lever  for  leveling  device. 
The  accessories  are  comprised  of — 

O. — 2  color  screens,  1  brush,  and  1  piece  of  chamois. 
The  carrying  case  equipment  (Plate  IX)  consists  of  the — 

W. — Range-finder,  case  with  tripod. 

X. — Range-finder  support. 

Y. — Tripod  mount  case. 
Optical  data : 

Invert  double-coincidence  type. 

Magnifying  power,  about  eleven  times. 

Actual  field  of  view  in  degrees,  4°. 

Entrance  pupil,  1  inch. 

Exit  pupil,  0.1  inch. 

Shortest  distance  measurable,  400  yards. 

(37) 


18 

The  accuracy  of  the  measurements  even  under  the  most  favorable 
conditions  will  be  unavoidably  influenced  by  a  known  error.  In  the 
following  table  the  lowest  values  of  these  errors  are  given,  but  when 
the  air  is  unsteady  or  the  target  is  unfavorable  these  errors  may  be 
increased  five  times. 

Yards.    [  Yards. 

400  yards 0.  7  j  900  yards 3.  9 

500  yards 1:1  I  1,000  yards 4.  4 


600  yards 1.  6 

700  yards 2.  2 

800  yards 2.  8 


1,600  yards 11 

2 ,000  yards 17 

3,000  yards 39 


OPERATION. 

The  range-finder  may  be  used  in  the  prone,  kneeling,  or  standing 
position.  The  spur  M  is  used  as  a  support  for  the  range-finder  when 
in  a  prone  position.  Where  the  soil  is  soft,  the  spur  is^pushed  as  one 
piece  vertically  into  the  ground.  For  use  .on  hard  soil,  the  three 
parts  composing  the  spur  are  spread  and  used  as  a  short  tripod. 
For  use  in  the  standing  position  without  the  tripod,  a  range-finder 
support  is  furnished  which  is  hung  on  the  observer's  shoulder  and 
holds  the  instrument  to  the  required  height. 

This  instrument  can  not  be  adjusted  to  the  vertical  position  on 
the  tripod.  For  use  with  the  tripod,  the  spur  is  folded  together,  in- 
serted in  the  casing  L  of  the  tripod,  and  clamped  securely.  For  use 
in  the  standing  position,  the  tripod  legs  are  spread  and  the  spur  can 
be  adjusted  in  the  casing  to  suit  the  height  of  the  observer's -eyes. 

ADJUSTMENTS. 

A.  ADJUSTMENT  FOR  HEIGHT. 

If,  when  in  sighting  at  an  object,  the  partial  images  do  not  touch 
with  similar  points  on  the  line  on  which  coincidence  is  to  be  made, 
the  instrument  is  out  of  adjustment  for  height.  To  correct  for  this 
error,  the  range-finder  must  be  trained  on  some  object  having  sharply 
defined  details  and  the  images  brought  exactly  opposite  each  other 
by  means  of  the  range  roller  (B),  Plate  VII.  The  revolving  cover  (C) 
is  moved  back  to  uncover  the  height  adjustment  roller  (D')  and  the 
images  of  similar  points  of  the  object  brought  equidistant  from  the 
dividing  line  upon  which  coincidence  is  to  be  made  by  turning  the 
roller  (D').  Both  images  may  then  be  made  to  touch  the  dividing 
line  simultaneously  by  turning  the  elevation  screw  (K).  The  height- 
adjustment  roller  should  always  be  covered  when  not  in  use. 

B.  ADJUSTMENT  FOR  DISTANCE. 

These  Bausch  &  Lomb  range-finders  have  no  adjustment  bar,  but 
are  provided  with  an  internal  adjusting  device.  The  adjustment  for 
correction  of  errors  in  distance  is  made  as  follows:  Turn  down  both 


19 

range-adjustment  levers  (R),  Plate  VIII,  until  vertical;  this  throws 
the  adjustment  marks  into  the  field  of  view.  The  locking  screw  on 
the  revolving  collar  (C),  Plate  VII,  marked  " Stadia  adjustment" 
is  then  released  and  the  collar  turned  until  the  mark  "  Stadia  adjust- 
ment" is  opposite  the  arrow  on  the  instrument.  The  range-adjust- 
ment roller  (D')  is  now  exposed.  The  range  scale  having  been  pre- 
viously set  to  read  infinity  the  single  adjusting  mark  is  brought 
exactly  in  the  middle  of  the  double  adjusting  marks  by  turning  the 
range  adjustment  roller  (D')  (see  fig.  11,  Plate  III).  The  position 
is  then  read  on  the  scale  (S),  Plate  VIII.  Should  there  be  any  varia- 
tion in  the  reading  of  the  scale  (S)  in  the  several  trials  the  final  ad- 
justment should  be  made  by  setting  the  scale  at  the  average  setting. 
The  revolving  cover  should  then  be  replaced  and  the  range  adjust- 
ment levers  turned  to  a  horizontal  position. 

MEASURING  RANGES. 

The  eyepiece  is  provided  with  an  adjustment  for  individual  correc- 
tion, equipped  with  a  diopter  scale.  If  this  correction  is  not  known 
it  should  be  determined  before  any  attempt  is  made  to  measure 
ranges.  The  determination  of  the  individual  correction  is  made  by 
turning  the  eyepiece  until  the  details  of  distant  objects  appear 
sharply  defined.  The  resulting  number  on  the  diopter  scale  should 
then  be  memorized  for  future  use. 

The  range-finder  having  been  set  approximately  on  the  target,  the 
instrument  is  leveled  by  loosening  the  locking  lever  (V).  Movement 
in  azimuth  is  accomplished  by  loosening  the  locking  screw  (I). 
Movement  in  elevation  is  accomplished  by  the  elevating  screw  (K), 
by  means  of  which  the  points  to  be  ranged  on  are  brought  to  the  coin- 
cidence line.  These  range-finders  being  of  the  double-coincidence 
type  have  a  field  divided  into  three  parts.  The  lower  field  is  erect, 
the  narrow  band  in  the  center  is  inverted  and  the  upper  field  is  erect. 
Coincidence  therefore  may  be  made  on  either  dividing  line.  These 
instruments  are  provided  with  an  "air"  and  " field''  adjustment  by 
which  the  position  of  the  inverted  image  may  be  shifted  to  make  co- 
incidence on  either  line  of  separation. '  When  the  knob  (H)  is  turned 
in  the  direction  of  the  arrow  marked  "field,"  coincidence  is  had  on 
the  lower  line  of  separation;  when  turned  to  the  arrow  marked  "air/7 
coincidence  is  had  on  the  upper  line.  By  turning  the  range-roller  (B) 
the  image  is  shifted  laterally  until  the  vertical  edge  or  points  range 
on  arc  exactly  above  one  another;  the  range  is  then  made  on  the 
scale. 

No  provision  has  been  made  in  these  mounts  for  rotating  the  range- 
finder  in  a  vertical  position. 


BAUSCH  &  LOME  RANGE-FINDER. 

(80  CM.  BASE.) 


EQUIPMENT  AND  GENERAL  DESCRIPTION. 

The  instrument  (Plates  X,  XI,  and  XII)  consists  of  the  range- 
finder  proper,  the  tripods  the  accessories,  and  the  carrying  cases. 

The  principal  parts  of  the  range-finder  proper  and  of  the  tripods 
and  mount  are: 

A  — Entrance  apertures. 
B  — Range  roller. 

C   — Height-adjusting  roller  (under  slide). 
D  — Range-correction  knob. 
F  — Eyepiece. 

G   — Range  scale  (under  slide). 
H  — Range-correction  reference  index. 
I   — Cover  for  ginible  joint  screw. 
J    — Internal  adjusting  levers. 
M  — Amber  and  smoked  glass  lever. 
O  — Eyepiece  cover. 
P   — Open  sight. 
E  — Instrument-carrying  strap. 
Short  tripod  and  mount: 
Q  — Releasing  plunger. 
R  —Tilting  knob. 
S   — Elevation  knob. 
T  — Azimuth  clamp  knob. 
N  —Spur. 
Long  tripod: 

W  —Tripod  legs. 
U  -  -Tripod  clamp  knob. 
V    -Tripod  casing  and  head. 
The  accessories  are  comprised  of — 
K  — Range  correction  key. 

1  brush  and  1  piece  of  chamois. 
The  carrying  case  equipment  consists  of— 

(a)  A  large  case,  with  straps,  for  range-finder  proper  and 
long  tripod.  A  container  is  provided  in  the  end  of 
this  case  for  the  accessories. 

(&)  A  small  carrying  case  (L)  for  tripod  mount  and  short 
tripod. 

(21) 


22 


Optical  data: 

Invert  double-coincidence  type. 

Actual  field  of  view  in  degrees,  4°. 

Power,  10.5. 

Entrance  pupil,  0.99  inch. 

Exit  pupil,  0.09  inch. 

Shortest  distance  measurable  400  yards. 

The  accuracy  of  the  measurements  even  under  the  most  favorable 
conditions  will  be  unavoidably  influenced  by  a  known  error.  In  the 
following  table  the  lowest  values  of  these  errors  are  given,  but  when 
the  air  is  unsteady  or  the  target  is  unfavorable,  these  errors  may  be 
increased  five  times. 


Yards. 

400  yards 1 

500  yards 1.25 

€00  yards... 2 

700  yards 3 

800  yards : 3. 8 


Yards. 

900  yards 4.9 

1,000  yards 6 

1,500  yards 12.5 

2,000  yards 23.8 

3,000  yards 54 


OPERATION. 

This  type  of  range-finder  can  be  used  in  the  prone,  kneeling,  or 
standing  position.  The  spur  is  used  as  a  support  for  the  range- 
finder  when  in  the  prone  position.  Where  the  soil  is  soft,  the  spur 
is  pushed  as  one  piece  vertically  into  the  ground.  For  use  on  hard 
soil,  the  three  parts  composing  the  spur  are  spread  and  used  as  a 
short  tripod.  For  use  with  the  long  tripod,  the  spur  is  folded  together 
and  inserted  into  casing  (V),  Plate  X,  of  .the  tripod,  and  locked 
securely  by  means  of  the  tripod  clamp  knob  (U).  The  spur  should 
be  adjusted  in  the  tripod  casing  to  suit  the  height  of  the  observer's 
eye.  The  instrument  can  also  be  used  in  the  prone  or  kneeling 
position  by  supporting  it  with  the  hands. 

ADJUSTMENTS. 

A.  ADJUSTMENT  FOR  HEIGHT. 

If,  when  in  sighting  at  an  object,  the  partial  images  do  not  touch 
the  dividing  line  with  similar  points,  so  that  one  image  reaches  the 
dividing  line  before  the  other  (fig.  6,  Plate  II),  the  instrument  is 
not  in  adjustment  for  height.  To  correct  this  error,  the  range- 
finder  must  be  trained  at  an  object  having  a  sharply  defined  hori- 
zontal line  or  particularly  prominent  point  and  the  images  brought 
laterally  exactly  opposite  each  other  by  means  of  the  measuring 
screw.  The  height-adjusting  roller  (c)  Plate  X  is  then  exposed 
by  moving  the  slide  marked  "halving  adjuster"  to  one  side  and  the 
adjustment  made  by  turning  the  roller  until  the  images  of  a  clearly 
defined  line  or  prominent  point  are  brought  equidistant  from  the 
dividing  line  in  each  field  (see  fig.  4,  Plate  II).  The  height  of  the 


23 

images  in  both  fields  is  controlled  by  means  of  the  elevation  knob 
(S),  Plate  X.  The  adjustment  made,  the  protective  slide  is  replaced 
to  guard  against  unauthorized  or  accidental  shifting. 

B.  ADJUSTMENT  FOR  DISTANCE. 

See  the  description  of  the  method  of  making  this  adjustment  for 
the  70-cm.  Bausch  &  Lomb  range-finder,  page  18. 

The  range  scale  should  be  set  to  read  infinity  before  making  this 
adjustment.  Bring  the  two  triple  mirror  prisms  in  use  by  pushing 
the  actuating  levers  (J),  Plate  XI,  downward.  The  illuminator 
for  the  adjusting  mark  is  opposite  the  eyepiece.  This  illuminates 
the  adjusting  mark  on  the  outward  prism  and  reflects  it  back  through 
the  ocular  prism,  right  telescope,  right  triple-mirror  prism,  left 
triple-mirror  prism,  left  telescope,  and  to  the  ocular  prism  again, 
where  this  reflected  mark  will  appear  -n  the  same  horizontal  plane 
as  the  index  slot,  and,  if  the  instrument  is  not  in  adjustment,  will 
not  be  equidistant  from  the  sides  of  the  slot.  Place  key  on  range 
correction  knob  (D),  Plate  X,  and  turn  until  the  reflected  mark  is 
equidistant  from  the  sides  of  the  index  slot.  (See  fig.  12,  Plate  III.) 
Observe  the  reading  on  the  range  correction  reference  index  (H), 
Plate  XI.  Repeat  this  operation  at  least  three  times  and  then  set 
index  (H)  at  the  mean  of  the  three  readings.  Throw  the  triple- 
mirror  prisms  out  of  the  field  by  means  of  the  actuating  levers  (J) 
and  remove  the  Adjustment  key.  The  instrument  should  now  be  in 
adjustment  for  reading  correct  ranges. 

MEASURING  RANGES. 

The  eyepiece  is  provided  with  an  adjustment  for  individual  cor- 
rection, equipped  with  diopter  scale  and  index.  If  this  correction 
is  not  known,  it  should  be  determined  before  any  attempt  is  made  to 
measure  ranges.  The  determination  of  the  individual  correction  is 
made  by  turning  the  eyepiece  until  the  details  of  distant  objects 
appear  sharply  defined.  The  resulting  number  on  the  diopter  scale 
should  then  be  ^memorized  for  future  use. 

Having  the  instrument  mounted  upon  either  tripod,  rotate  the 
protective  bands  so  as  to  expose  the  entrance  apertures  (A),  release 
azimuth  clamp  (T),  and  turn  the  range-finder  toward  the  target. 
Tilt  the  range-finder  until  approximately  horizontal  by  means  of  the 
tilting  knob  (R),  and  look  for  the  target  in  either  open  sight  (P), 
using  the  elevation  knob  (S)  to  elevate  or  depress  the  line  of  sight  as 
necessary. 

These  range-finders  being  of  the  double-coincidence  type  have  the 
field  divided  into  three  parts.  The  lower  field  is  erect,  the  narrow 
band  in  the  center  is  inverted,  and  the  upper  field  is  erect.  Coinci- 
dence, therefore,  must  always  be  made  on  the  lower  dividing  line. 


24 

> 

Having  placed  the  image  of  the  target  in  the  center  of  the  field  of 
view,  elevate  or  depress  the  line  of  sight  by/tneans  of  the  elevation 
knob  (S)  until  similar  points  of  the  target?,  in  the  upper  and  lower 
fields,  are  equidistant  from  the  lower  dividing  line.  Turn  the  range 
roller  (B),  Plate  X,  until  similar  points  of  the  image  in  the  central 
field  are  exactly  above  the  same  points  in  the  lower  field.  (See  fig.  5, 
PL  II.)  The  range  is  then  read  on  the  range  scale. 

With  targets  which  have  no  vertical  points  or  lines  but  show  hori- 
zontal lines,  such  as  tops  of  heights,  intrenchments,  etc.,  the  ranges 
are  measured  on  the  horizontal  line  by  holding  the  instrument 
vertical. 

In  this  connection  see  also  parag»aph  " General  instructions/7 
11  of  this  pamphlet. 


CARE  AND  PRESERVATION. 


Under  no  circumstances  shall  any  range-finder  be  disassembled  or 
adjusted  except  by  a  skilled  mechanic  upon  order  by  an  Ordnance 
Officer  or  by  an  arsenal  especially  equipped  for  instrument  work. 
The  interior  is  sealed  against  dust  and  moisture  and  any  disassembling 
will  unseal  the  internal  parts  and  may  cause  serious  damage  to  the 
instrument.  Every  precaution  should  be  taken  to  preserve  the  range- 
finder  from  -  unnecessary  injury.  It  should  not  be  thrown  on  the 
ground  or  subjected  to  any  avoidable  shocks,  strains,  neglect,  or  other 
ill  usage.  To  care  for  the  instrument  properly  those  responsible  for 
it  should  familiarize  themselves  with  the  functions  of  its  various 
parts,  the  methods  of  adjusting,  and  the  proper  method  of  caring  for 
the  mechanical  and  optical  parts. 

The  instrument  should  be  kept  in  a  dry  place.  The  working  parts 
should  be  oiled  regularly  though,  sparingly  with  oil  issued  by  the  Ord- 
nance Department.  Proper  care  should  be  taken  that  no  oil  drops 
upon  or  reaches  the  optical  surfaces  or  other  parts  which  do  not 
require  oiling. 

The  range-finder  should  be  protected  as  much  as  possible  in  wet 
weather.  When  rain  ceases  or  operations  are  over  the  outside  of  the 
range-finder  should  be  wiped  off  with  a  cloth. 

The  end  windows,  the  scale  window,  and  the  eyepiece  should  be 
wiped  with  a  piece  of  clean  chamois  skin  especially  kept  for  the  pur- 
pose. Dry  dust  is  removed  with  the  brush. 

The  metal  work  should  be  rubbed  down  with  a  dry  cloth;  cleaning 
paste  must  not  be  used  on  any  part  of  the  instrument. 

The  greatest  care  should  be  taken  not  to  allow  any  oil  or  grease  to 
get  on  any  of  the  windows,  lenses,  or  prisms,  nor  should  they  be 
touched  with  the  fingers. 

If  in  any  case  moisture  is  found  collected  on  the  inner  surface  of  the 
windows  or  the  internal  optical  parts  the  end  plates  to  which  the 
padded  buffers  are  attached  should  be  removed  for  a  short  time  to 
allow  a  current  of  air  to  pass  through  the  tube,  but  this  should 
only  be  done  in  dry  weather,  never  in  rain  or  fog,  or  if  smoke  or  dust 
is  about  the  range-finder. 

All  work  on  the  range-finder,  such  as  unnecessary  turning  of  screws, 
etc.,  not  incident  to  its  use  or  cleaning,  is  strictly  forbidden. 

WAR  DEPARTMENT, 

OFFICE  OF  THE  CHIEF  OF  ORDNANCE, 

Washington,  December  9,  1915. 

Form  No.  1797. 

Ed.  June  16-17—8,000. 

(25) 

0 


DIAGRAM  TO  ILLUST&rTC  THE 

PRINCIPLE:  or  RANGE  ~FINDEKS, 


PLATE   II. 


FIG.  3. 


FIG.  4. 


FIG.  5. 


FIG.  6. 


FIG.  7. 


FIG.  8. 


PLATE  III. 


FIG.  9. 


FKS.II. 


FIG.  10 


PLATE  V. 


It.. 


GOERZ,  80  CM.  BASE,   RANGE  FINDER.     FRONT  VIEW. 


PLATE  VI. 


GOERZ,  80  CM.  BASE,   RANGE  FINDER.     REAR  VIEW. 


PLATE  VIM. 


fcBAUSCH   &  LOMB,  70  CM.  BASE,   RANGE  FINDER.     REAR  VIEW. 


PLATE  X. 


6 


BAUSCH   &   LOMB,  80  CM.   BASE,   RANGE   FINDER.     FRONT  VIEW. 


PLATE  XI, 


BAUSCH   &   LOME,  80  CM.   BASE,   RANGE   FINDER.     REAR   VIEW. 


PLATE  XII, 


GOERZ,  80  CM.   BASE,    RANGE-FINDER   CARRYING   CASE. 


BAUSCH   &   LOMB,  80  CM.   BASE,   RANGE   FINDER,   WITH   SHORT  TRIPOD. 


